1. Field of the Invention
This invention relates to a circuit which is arranged to generate a reset signal when the output of a power source comes to a stop.
2. Description of the Related Art
The conventional reset signal generating circuit of the above stated kind has been arranged to produce a reset signal which becomes active or enabled (i) until the lapse of a given period of time after a power supply is switched on; (ii) until the lapse of a given period of time after the voltage of power supply has reached a given value; or (iii) when the power supply voltage drops to a value lower than a given value.
FIG. 1 of the accompanying drawings is a wave form chart showing the operation of a reset signal generating circuit of that kind. As shown, the voltage of the power supply gradually rises when a power supply control signal which indicates switching on of the power supply changes from an off-state to an on-state. At this time, the reset signal is in an active state. A timer which is not shown begins to operate when the supply voltage comes to exceed a given detection level. Then, after the lapse of a period of time T, the state of the reset signal becomes inactive . In other words, a reset state is canceled when the power supply voltage is sufficiently stabilized.
The power supply voltage begins to gradually drop when the power supply control signal conversely changes from the on-state to the off-state. However, the reset signal remains in the inactive state until the supply voltage becomes lower than the detection level. The reset signal becomes active when the power supply voltage becomes lower than the detection level.
With a level detecting circuit arranged in combination with a timer in this manner, a reset state can be canceled only after the power supply voltage reaches a sufficient value. When the power supply is turned off, however, the reset signal becomes active after the power supply voltage becomes lower than a given level. This, therefore, tends to cause some error to take place within the body of a system.
Generally, the error of this kind does not present any serious problem as it takes place immediately before the power supply turns off. However, it tends to become a serious problem for a system of the kind arranged to store some necessary data in a backup memory, an E.sup.2 PROM (electrically erasable programmable read-only memory) or the like.